17 small catchment research in the evaluation and ......reviewed by keller (1988) and the evaluation...

17 Small Catchment Research inthe Evaluation and Developmentof Forest Management Practices

WAYNE T. SWANK AND CHRIS E. JOHNSON

17.1 INTRODUCTION

The hydrologic literature is replete with examples of the use of small catchmentexperiments to examine the effects of forest management practices on waterresources. Examination of hydrology symposia in recent years indicates thatapproximately 20% of the papers deal with some aspect of land-use activities andhydrologic interactions. It is not our intent in this chapter to review this volumi-nous literature, but rather: (1) to provide briefly a historical background on thetopic; (2) to outline some philosophical and conceptual approaches in using smallcatchments to evaluate forest management activities; (3) to illustrate the benefitsand limitations of these approaches through select examples; and (4) to suggesttopics considered to be of high priority for future research.

17.2 HISTORICAL BACKGROUND

The roots of forest hydrologic investigations are embedded in basic questions ofthe relationship between forests and runoff. The historical development of smallcatchment research in a number of countries is provided in the formative volumeInternational Symposium on Forest Hydrology (Sopper and Lull, 1967), the prod-uct of a seminar held at the Pennsylvania State University in 1965. Forest hydro-logic investigations have a long history and extend over more than a century. Forexample, two experimental watersheds were established in Czechoslovakia in1867 in the upper reaches of the River Becva in Moravia to examine the roleforests play in surface runoff (Nemec et ai., 1967). In later years, controversy overthe hydrologic influence of forests on floods led to establishment of additionalexperimental watersheds (Valek, 1962). In 1902, paired experimental watershedswere established in the Emmental region of Switzerland to study the effects ofdeforestation and intensive use of mountain lands on flood flows and sedimenttransport (Keller, 1988). Subsequently, in the United States, concern about soil

Biogeochemistry of Small Catchments: A Tool for Environmental ResearchEdited by B. Moldan and J.Cemy@ 1994 SCOPE Published by John Wiley & Sons Ltd

.r.~';),au fA~~

384 BIOGEOCHEMISTRY OF SMALL CATCHMENTS

erosion, flood control, sustained streamflow and dwindling timber resources ledto establishment of forest reserves (National Forests) from the public domain landsin the West in the early 1900s. In fact, enactment of the Weeks Act of 1911 wasbased on the role of forests in regulating the flow of navigable streams.

The first catchment experiment in the United States was initiated in 1909 tomeasure streamflow before and after removal of trees at Wagon Wheel Gap inColorado (Bates and Henry, 1928). The need for scientific studies of factors con-trolling floods and erosion was accentuated by the disastrous 1927 flood in theMississippi River Basin. Attendant controversy over the role of forests in regulat-ing streamflow led to formal establishment of watershed management research bythe USDA Forest Service. For example, the Coweeta Basin in North Carolina wasselected as a site for a hydrologic laboratory in 1931. As reviewed by Douglassand Hoover (1988), the research approach at Coweeta encompassed the hydrologiccycle with studies of basic hydrologic processes on individual experimental basinsto determine the principles underlying the relation of forests to the supply and dis-tribution of water. This approach formed the template for other Forest Servicewatershed management programmes which followed in the period 1940-1955. Therecent development of small catchment programmes in Europe has been partiallyreviewed by Keller (1988) and the evaluation of forest hydrology in Europe issummarized by Brechtel (1982). Many projects were implemented in the period1960-1980 including a number of European catchments studies in connection withforest dieback, namely in Germany.

Indeed, the development of experimental watersheds as a research tool was adirect result of the need to understand the influence of forests and related manage-ment activities on water resources. Controversy about the influence of forests onfloods and concern about erosion frequently provided the stimulus to developresearch programmes.

17.3 CONCEPTUAL CONSIDERATIONS

A basic premise of many experimental watershed programmes is that human activ-ities on the landscape ultimately have some influence on the water resource. It isuniversally possible to examine the use or misuse of land by measuring the proper-ties of surface or subsurface waters draining forest ecosystems. Thus, alteration ofthe quality, quantity, or timing of water flowing from the land can be viewed as asensitive indicator of the long-term success or failure of land management pro-grammes (Hewlett, 1964). An equally important tenet of small catchment researchis that good resource management is synonymous with good ecosystem manage-ment (Swank and Crossley, 1988a). The utility of catchments as a basic unit forexamining human- induced effects on forest structure and function has beendemonstrated in many regions of the world. In all cases, water is a common threadsince it is the primary mechanism for transporting substances within and fromforested lands.

FOREST MANAGEMENT PRACTICES 385

The design of small catchment studies has emerged from a variety of forestmanagement perspectives including approaches that: (1) demonstrate the deleteri-ous or beneficial effects of prevailing land-use activities or the effects on waterresources of different (improved) forest management technologies; (2) elucidateprinciples of forest hydrologic processes; and (3) meld theory development, exper-imental testing, modelling and subsequent application of knowledge to manage-ment. Obviously, the approach and design of catchment experiments are, or shouldbe, based on specifically stated questions or hypotheses.

The use of small catchments to demonstrate the integrated effects of specificforest management activities on water resources is a very effective approach toaddress local or regional issues. The demonstration approach usually entails asequence of "this is what we had", "this was the management activity", and "thiswas the result" (as measured downstream of the activity at a weir or gauging sta-tion). Such an approach can provide both a visual and a quantitative assessment ofhydrologic responses to management of sufficient magnitude to either alter or sup-port current practices and provide guidelines for managers and policy-makers.Examples include the demonstration of the effects of alternative logging methodson erosion and stream sedimentation; effects of mechanical site preparation on soilloss; changes in streamflow quantity and timing following forest cutting, speciesconversion, or alternative land-use practices (i.e. forest to agriculture conversions);and the impacts of pesticide or fertilizer applications on water quality. The litera-ture is replete with examples of these practices and others which provide findingsuseful for forest resource planning. However, small catchment demonstrations arefrequently case histories that are site-specific. Cause-and-effect relationships basedon process studies within the catchment may be lacking and extrapolation to othercatchments limited. Thus, a major disadvantage of catchment demonstrations istheir lack of quantitative prediction and potentially high risk in extending results toother ecosystems.

Small catchment research has been quite successful in establishing hydrologicprinciples which are essential to interpreting the effects of management practices.Examples include elucidation of mechanisms of water movement down forestslopes; quantification of various components of evapotranspiration and effects onrunoff generation; derivation of functional relationships between forest standstructure, evapotranspiration, and runoff; elucidation of the importance of the dif-ferent forms of precipitation on the quantity and timing of runoff; and quantifica-tion of differences in hydrologic processes between different forest types.Enhanced understanding of principles provides a basis for predicting and interpret-ing hydrologic responses in relation to a variety of management prescriptions.

Perhaps the greatest management benefits to be derived from catchment experi-ments are from studies that link sound theoretical development with appropriateexperimental testing, including process-level research conducted concurrently withthe management prescription (Figure 17.1). This design encompasses the otherapproaches and frequently leads to the modification and/or development of modelsthat are testable at the catchment scale. Thus, the demonstrated effects of a


Theoretical Basis

/ (hypothe.e., model.) ~Catchment Experimentation ~ . Management(testing, principles, (applications, guidelines)

demonstrations)

Figure 17.1 Small catchment research yields the greatest benefits when linked with asound theoretical basis for conducting the experiment. The relevance of theoretical con-structs is improved through an understanding of management needs, objectives and forestplanning procedures. In turn, the results of catchment experiments are used to revise thetheory and, as appropriate, to provide guidelines and improved methods for resource man-agement.

management practice on the water resource can be related to causative factors,providing a sound basis for the prediction and extrapolation of the magnitude ofresponses in other catchments. This design of catchment studies is an interdisci-plinary, integrated approach and usually requires substantial resources to imple-ment. However, it is frequently the best approach to answer complex questionsabout management effects on forest ecosystems.

17.4 SELECT EXAMPLES OF APPROACHES

Below we will illustrate the usefulness of small catchments in assessing the effectsof various forest management activities on water and soil resources. Many exam-ples could be cited from the literature and we have selected only a few that addressthe topics of water yield, timing of streamflow, water quality parameters, andnutrient cycling with a focus on soil nutrients and forest productivity.

17.4.1 WATERYIELD AND TIMING OF STREAMFLOW

17.4.1.1 General

Transpiration, interception and, hence, evapotranspiration are generally reducedwith forest harvesting, which produces more soil water available for the remainingplants and/or increased water movement to streams or groundwater. The quantityof extra water produced depends on a combination of factors, including theamount and type of forest vegetation, intensity and pattern of cutting and climateof the area. Conversely, establishment of forest cover on sparsely vegetated landgenerally decreases water yield. These generalizations are based on a review ofresults from 39 catchment experiments throughout the world (Hibbert, 1967). The


addition of 55 experiments to the data base (Bosch and Hewlett, 1982) supportthese generalizations. It was additionally inferred from the world literature that theinfluence of manipulated vegetation on water yield was greatest for conifers, fol-lowed by deciduous hardwood, brush and grass cover (Bosch and Hewlett, 1982).Water yield increases due to cutting or decreases due to planting are largest in highrainfall areas. However, clearcutting effects are shorter-lived than in low rainfallareas because vegetation regrowth is more rapid.

17.4.1.2 Regional examples

Swank et al. (1989) provide more specific, quantitative relationships between for-est cover alternatives and water yield responses in a summary of effects of timbermanagement practices on soil and water for major forest types in the UnitedStates. For example, from catchment experiments involving a variety of cuttingintensities in hardwood forests of the Appalachian Highlands Physiographic regionof the eastern United States, a generalized response of first-year streamflowincreases after cutting is available (Figure 17.2; Douglass and Swank, 1975). Firstyear increases are proportional to the reduction in basal area and also related to theenergy available for evapotranspiration (insolation index). Thus, clearcutting anorth-facing (0.22 index) hardwood stand increases streamflow about 41 cm whileselection cutting in the same stand (e.g. 30% of basal area) increases flow about 8cm. These generalized relations apply to a major portion of the moist eastern hard-wood region and can be refined by considering soil depth and cutting patterns(Swank et ai., 1989). Streamflow increases are typically largest in the first yearafter cutting and decline as vegetation regrows. Streamflow returns toward base-line levels at a rate approximating a logarithmic decay curve and the average dura-tion is commonly six to ten years (Swift and Swank, 1981).

Equations for predicting water yield responses to silvicultural prescriptions,based on these catchment results, are available for the region (Douglass andSwank, 1975) and have frequently been used as a tool in forest management plan-ning. Their usefulness has been further tested in a multiple-use demonstrationexperiment at Coweeta Hydrologic Laboratory in western North Carolina(Douglass and Swank, 1976). A 144-ha catchment was selected to pilot test water,timber, wildlife and recreation objectives. Silvicultural prescriptions included thin-ning 39 ha of the highly productive mesic cove forest; clearcutting 77 ha of mid-and upper slopes to regenerate mixed oak forests that had previously been high-graded; and 28 ha of the highest elevation portions of the forest were preserved toprotect slopes, provide wildlife cover and provide an attractive setting for a majorhiking trail. The cumulative effects of these timber prescriptions on water yieldwere estimated using the equations previously described and values were com-pared with measured responses for the entire catchment (Table 17.1). In the earlyyears following harvest, predicted changes were slightly greater than measuredvalues; in later years, the model under- predicted water yield. Year-to-year

388

45

40

35

Eu- 30~a-'LL

~ 25w0::I-(f)

z 20

w(f)<{

~ 15uz

BIOGEOCHEMISTRY OF SMALL CATCHMENTS

10

FOR APPALACHIANHIGHLANDS

5

010 20 30 40 50 60 70

REDUCTIONIN BASAL.AREA ("!o)80 90 100

Figure 17.2 Streamflow increases in the first year after cutting a mixed hardwood forestare related to the amount of vegetation cut (expressed as basal area) and the amount of solarradiation received by the forest (expressed as insolation index). The relationships werederived from experimental catchment studies in eastern hardwood forests of the UnitedStates (from Douglass and Swank, 1975).

variability in measured responses is strongly influenced by precipitation amountsand distribution, factors not accounted for in prediction equations. As the har-vested forests regrew, water yield increases declined to no significant difference inyear nine. For the entire response period, there was little difference between mea-sured and predicted yield increases, with a total yield change of 67 em. Thus, suchequations derived from experimental catchments provide predictions of sufficientaccuracy for most planning purposes.

Small catchment experiments have also shown that forest management activitiessuch as species conversions can produce dramatic changes in water yield.Conversion of mixed hardwood forests to white pine plantations in the southern


~~

~ -10!~ -20

Table 17.1 Comparison of predicted changes in annualwater yield with measured values for Coweeta WS 28, a144 ha mixed hardwood forest with a combination of thin-

ning, clearcutting, and no cutting

105

WATERSHED 1

0

t -5

\20 Years After

WATERSHED 17 Planting

/

-1010 Years After

Planting

IIII

5 1&1~U

0 !

-5

-10...~ -30;:)I-~ 1957 59 61 63 65 67 69 71

MAY-APRIL WATER YEARS

73 75 77 79 81 83

Figure 17.3 Annual changes in streamflow on two Coweeta catchments followingclearcutting and conversion from mixed hardwood to white pine (from Swank et at., 1988).About 10 to 12years after planting, evapotranspiration was greater from the pine than fromhardwood and, hence, streamflow was less than predicted for mature hardwoods.

:to 0......

-101&1Q: 30I-III

Q 201&1I-U 10-Q1&1Q: 0Q.

Year following Measured Predictedharvest change change

(cm year-I) (cm year-I)

I 16.5 18.32 10.2 13.03 7.9 9.94 2.3 7.65 2.8 6.16 8.1 4.67 lOA 3.68 8.6 2.39 1.0 1.5

Total 67.8 66.9


AppalachianMountainsreducedwater yield only 10yearsafter planting (Swankand Douglass, 1974). By age 15, water yield reductions were about 20 cm (20%)less than expected for a hardwood forest (Figure 17.3). Reasons for greater evapo-rative losses from young pine than from mature hardwoods have been described inseveral reports (Swank and Miner, 1968; Swank and Douglass, 1974; Swift et aZ.,1975). In another experiment, hardwoods were converted to a grass cover(Hibbert, 1969). Water yield response varied from no significant change the firstyear after conversion to an increase of 14 cm in the fifth year as grass productiondeclined. Vegetation conversions and comparisons on experimental watershedselsewhere have also shown large changes in water yield (Feller, 1981; Pearce andRowe, 1979;Dons, 1986).

It is apparent that findings on water yield from small catchment experimentsprovide general guidelines for forest and water managers. In fact, in some regionswhere numerous experiments have been conducted over a range of conditions,quantitative relationships are available to predict water yield responses to silvicul-tural activities. However, it is difficult to extrapolate single and even multiplecatchment results in both time and space with a high degree of confidence andmethods are needed that realistically link cause-and-effect relationships.

17.4.1.3 Hydrologic models

Modelling provides a logical method for linking forest management catchmentexperiments with process-level studies. PROSPER, an evapotranspiration simula-tion model is one example of a hydrologic model developed from and applied toforest catchment studies. The model links atmosphere, vegetation and underlyingsoils through use of simultaneous equations that combine energy balance, masstransport and soil moisture dynamics (Goldstein et aI., 1974; Huff and Swank,1985). The model has been used to evaluate the consequences of thinning,clearcutting, species conversions and long-term forest succession (Swift et aZ.,1975; Troendle, 1979; Huff and Swank, 1985). Simulations of evapotranspirationin the Southern Appalachians for mature mixed hardwoods, clearcut hardwoodsand white pine plantations showed large differences in timing and water use rates

Table 17.2 Change (cm) in annual streamflow attributes due to conversion from matureoak-hickory forest (from Swift et at., 1975)

Water yearMay-April

1940/41I94l!421963/641971!721972/73

Annualprecipitation

White PineMeasured Simulated

streamflow drainage

ClearcutMeasured Simulated

streamflow drainage

+36.0+41.3+38.1

154.0158.5196.0198.9234.3

-20.2-18.3

-20.2-16.9

+36.5


15

§~

".2elO's."!!'0

tIII

"V

.!..!!5,.

~

0 MJ JASONDJ FMAMJ JASONDJ FMA1971 1972 1973

Figure 17.4 Monthly totals of simulated evapotranspiration for a 16-year-old white pineplantation, a mature oak-hickory forest, and a clearcut forest (from Swift et at., 1975).

of the cover type (Figure 17.4). Differences in water use could be attributed toalteration of interception and transpiration (Swift et aI., 1975). Annual simulateddrainage associated with silvicultural practices of conversion from mixed hard-woods to white pine and clearcut forest showed good agreement with measuredcatchment responses (Table 17.2). The model has also performed well for otherforest types in different regions of North America (Voseand Swank, 1992). PROS-PER and another streamflow simulator (WATBAL)were further modified to eval-uate the impact of silvicultural activities on the hydrologic cycle; subsequently,these methods were incorporated in a handbook to guide best management prac-tices for forest resources (Troendle, 1979).

17.4.1.4 Timing of streamflow

Paired watershed experiments provide a basis for determining the effects of man-agement practices on the intra-annual distribution of streamflow and storm hydro-graph characteristics. Annual water yield increases associated with cutting(Section 17.4.1.2) are not distributed uniformly throughout the year. In areas withshallow soils and a snow cover, a large proportion of increased flow occurs in thespring and summer months in direct response to precipitation patterns and evapo-transpiration reductions due to cutting. On catchments with deep soils and rain-dominated precipitation, flow increases are observed in the summer and autumnmonthsand may extend into early winter (Figure 17.5).


COWEETA17MAY-APRIL WATERYEAR

15

14

E 13-912~ 11

g 10LL~ 9« 8Wc:: 7I-(j) 6

~ 5« 4c::W 3~ 2

1

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~ Mean Flaw

D Increase

c\ ~ c\ -,Co '" '" ,.. /.(; '" /0 ~ ~~'?- -i5 "i'v <$>'e;<:}0(; ",° <0v ',,'?-~v ~'?' .,;?~

MONTH

Figure 17.5 Mean monthly flow on Coweeta WS 17 prior to cJearcutting and meanmonthly increases during a seven-year period of annual recutting (from Swank et ai., 1988).

Reduced evapotranspiration after cutting means less potential storage of soilwater during storms which, in turn, contributes to peak flow rates and stormflowvolumes. Harvesting has the minimum impact on the storm hydrograph in the win-ter months when both cut and uncut catchments are fully recharged (Lull andSopper, 1967). Cutting causes more rapid snowmelt in the spring which mayincrease peak flow rates (Hornbeck, 1973; Verryet ai., 1983). The type of harvest-ing method and associated soil compaction from logging roads and skid trails isthe major factor that increases stormflow (Harr et al., 1979). For example, com-merciaI logging in the Southern Appalachians increases quickflow volumes 10 to20% and peak flow rates 15 to 30% and there is little change in other storm hydro-graph parameters (Table 17.3). Knowledge about such changes is useful in design-ing culvert sizes but the consequences of increases further downstream areconsidered to be insignificant since the proportion of a drainage basin cut within ayear is usually small.

17.4.2 WATERQUALITY

Catchment experiments have provided important insights into the magnitude of theeffects of forest management activities on water quality. Characteristics mostaffected are sediment load, dissolved nutrient concentrations and temperature.

Nt A. not available; NS. not significant.

AInclusive years after treatment used for total quickflow and peakflow rates, respectively.

\H\0\H

Table 17.3 Summary of changes in storm hydrograph parameters after clearcutting and/or commerical harvest in mixed hardwoodforests at Coweeta Hydrologic Laboratory

Mean stormIncreases for selected hydrograph parameters

TotalInclusive quickflow Peakflow Initial flow Recession

Watershed Size years after volume rate rate timenumber (ha) Treatment treatment (%) (%) (%) (%)

7 58.7 Clearcutting, cable logging, 3 10 15 14 10product removal and minimalroad density

28 144.1 77 ha clearcut, 39 ha 9,2Q 17 30 N/A N/Athinned, 28 ha no cutting,products removed; high roaddensity

37 43.7 Clearcut, no products 4 11 7 NS NSremoved


Changesin theseparametersvarywidely dependingon thepropertiesof thebase-line forest ecosystem and the type of management activity, such as harvesting andassociated logging methods; site preparation methods such as use of mechanicalequipment; and stand improvement between initial re-establishment and harvest-ing which may involve the use of fire, herbicides, or fertilizer.

17.4.2.1 Sedimentation

A major concern in harvest and regeneration practices is the impact on stream sed-imentation. Roads and skid trails are the primary sources of sediment associatedwith silvicultural activities as documented in many catchment studies (Lull andReinhart, 1972; Anderson et ai., 1976; Swift, 1988). Careful layout, constructionand maintenance of roads will minimize increases in stream turbidities. The mostsensitive aspect of road construction occurs at stream crossings because soil fromcuts and fills has direct access to the stream. Rapid establishment of surface pro-tection such as a grass cover is a key factor in erosion control for roads. Asdemonstrated in catchment studies at Coweeta, newly constructed roads lose themost soil in the short period before grass becomes established (Figure 17.6). Inthe example shown, about 75% of the soil eroded during the 2.5 years of observa-tion was delivered to the stream immediately below a road crossing in the first two

800

600

,".c~II)II)

g 400...J0II)

200

CUMULATIVESOIL LOSS

SAMPLER 702

SMALL STREAM BELOW ROAD

BASED ON AREA OF ROADWAY

WHICH DRAINS DIRECTLY INTO STREAM

0M J J A SON D J F M A M J J A SON D J F M A M J J A S 0

1976 1977 1978

Figure 17.6 Cumulative soil loss from a forest road at a stream crossing during the first2.5 years after start of construction (from Swift, 1988).


months. Long-tenn research at the catchment scale has provided practical guidelinesfor protecting water quality related to forest access road construction (Swift, 1988).

17.4.2.2 Stream temperature

Catchment experiments have shown that removal of the forest canopy adjacent toforest streams increases solar heating of surface waters with a consequent increasein maximum summer temperatures of 1.1 to 6.1°C in eastern United States streams(Swift and Messer, 1971; Hornbeck and Federer, 1975; Kochenderfer andAubertin, 1975). In western United States forests, even larger stream temperatureincreases have been observed following harvest of streamside vegetation (Brownand Krygier, 1970). However, catchment studies have shown that adverseincreases in stream temperature can be avoided by maintaining a buffer strip alongstreams (Brown et ai., 1971; Hornbeck et ai., 1986). As Stone et ai. (1980)observed, elevated stream temperature is not an inevitable consequence of the har-vest method but is subject to trade-off analysis in forest planning.

17.4.2.3 Dissolved nutrients

Forest management activities such as forest cutting and harvesting interrupt thenatural recycling of nutrients and there is concern that nutrients released mayaffect downstream uses or reduce site productivity. The use of small catchments inrecent decades as a tool for biogeochemical cycling studies has produced a largebody of infonnation on streamwater nutrient responses to management, particu-larly clearcutting. Changes in streamwater nutrient concentrations following cut-ting vary substantially between localities, even within a physiographic region. Forexample, in Central and Southern Appalachian forests, measurable increases inconcentrations of N03-, K+, and other constituents have been observed followingcutting but the magnitude of change is small and unimportant to downstream uses(Swank et ai., 1989). In contrast, clearcutting in northern hardwood forests mayresult in large increases in concentrations of some nutrients (Hornbeck et ai.,1986). Process research on catchments has identified some of the reasons for avaried ecosystem response to disturbance.

Small paired catchment experiments provide a basis to assess effects of man-agement practices on dissolved nutrients (Figure 17.7). Dissolved nutrient concen-trations should be measured before, during and after disturbance on reference andmanipulated catchments. Pretreatment calibration regressions of monthly fluxes ofnutrients between the reference and managed catchments can be used to estimatethe magnitude of management impacts (Table 17.4). In the illustration used, con-centration changes were small for most nutrients but when combined with annualincreases in discharge due to cutting, there was a substantial increase in nutrientexport (Figure 17.7, Table 17.4). These accelerated nutrient losses may be morecritical to soil fertility than water quality as discussed below.


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MONTH-YEAR

Figure 17.7 Mean monthly concentrations (flow-weighted) of K, Ca and N03--N instreamwater of Coweeta WS7 during calibration (1971-76), road construction, c1earcuttingand logging, site preparation, and post-harvest recovery periods (Swank, 1988).

17.4.3 ELEMENT CYCLING

In any ecosystem, elemental inputs through atmospheric deposition and humanactivity (e.g. fertilization) must balance hydrologic outputs and changes in internalstores. The construction of element "budgets" has been the focus of numerousinvestigations (Chapter 8). Aggrading forest ecosystems are often characterized byrelatively low elemental inputs and relatively high rates of biomass uptake (e.g.Likens et ai., 1977; Monk and Day, 1988; Johnson and Todd, 1990). Thus, anyforest management activity altering existing element balances may adverselyaffect the regenerative capacity of forest stands.

Of all management activities, logging is perhaps the most disruptive to elementcycles. In many forests, stores of nutrient elements in forest biomass are substan-tial in comparison to the available pool in the soil. Removal of biomass by loggingcan therefore represent a major loss of nutrients from the ecosystem.

....\0

:J

Table 17.4 Annual changes in streamflow and solute export following cIearcutting and logging on Coweeta Watershed 7

Time sincetreatment Increases or decreasesQin solute export (kg ha-l)

(May-April Flow N03-N NH4-N P04 K Na Ca Mg S04 CIwater year) (cm)

First 4 months 0.5 0,01 <0.01 0.04 0.43 0.42 0.24 0.26 1.17 0.68First full year 27.3 0.26 0.03 0.12 1.98 1.37 2.60 0.96 0.81 1.13Year 2 23.8 1.12 <0.01 0.03 1.95 2.22 2.51 1.15 -0.24 1.62Year 3 13.1 1.27 0.05 0.06 2.40 2.68 3.16 1.42 1.16 2.08Year 4 8.6 0.25 0.15 0.06 0.80 1.07 1.63 0.46 0.93 0.59YearS 8.0 0.28 0.01 0.02 0.52 0.13 1.19 0.18 0.11 0.10

"Increaseor decreasebasedon monthlycalibrationregressions.


The small watershed method has proved to be a useful tool for studying theeffects of logging and other forest management activities on element dynamics inforests (see reviews by Mann et ai., 1988; Vitousek and Mellilo, 1979; Swank andCrossley, 1988b). The use of small watersheds offers the researcher a defined areain which intensive measurements of biomass and soil element pools can be confi-dently related to input-output budgets. By monitoring changes in element budgets,the small watershed method can be used to determine the effects of managementpractices on nutrient flows and capital, the mechanisms by which nutrient storesare reorganized following large-scale disturbance and long-term estimates of nutri-ent depletion rates.

17.4.3.1 Case study: Hubbard Brook

Results from an intensive study at the Hubbard Brook Experimental Forest(HBEF), in central New Hampshire, USA, provide a good example of the utility ofthe small-watershed approach. Prior to whole-tree clearcutting, watershed 5 (W5)at the HBEF was dominated by hardwood forest vegetation. Various pools of N,Ca, K and Mg in biomass and soil prior to clearcutting are given in Table 17.5.The whole-tree harvest removed 88% of the above- ground biomass. The har-vested biomass accounted for 6.1%,180%,141% and 116% of the soil pools ofavailable N, Ca, K and Mg, respectively.

Post-harvest streamwater solute concentrations from W5 (Figure 17.8;Lawrence et al., 1987) showed patterns similar to previous deforestation experi-ments at the HBEF (Likens et ai., 1970; Hornbeck et ai., 1986; Martin et ai.,1986). The effluxes of solutes in W5 streamwater following whole-tree clearcut-ting are not yet available. Thus, estimates for hydrologic losses following cuttingare based on results of similar clearcut watersheds in the region (Martin et ai.,1986). Alteration of ecosystem processes that cause the observed trends in stream

Table 17.5 Nutrient content (kg ha-I) of biomass and soils on W5 ofthe Hubbard BrookExperimental Forest and nutrient removal in whole-tree harvesting. Based on data on for-est floor and soil N from Huntington and Ryan (1990), on forest floor and soil Ca, K, Mgfrom Johnson et ai. (1991) and unpublished data ofT.G. Siccama, Yale School of Forestry

"Estimated using ratio of above-ground: below-ground biomass content from Likens et al. (1977).bMineral soil N is total N. Mineral soil Ca, K and Mg are exchangeable.

N Ca K Mg

Above-ground biomass 506 656 245 58Below-ground biomassa 261 173 100 21Forest floor total 1340 128 61 17Mineral soilb 5900 193 92 27Removed in harvest 445 578 216 51Uncut 20 26 10 2Residual biomass (slash) 40 53 20 5

FOREST MANAGEMENT PRACTICES

1986

Date

Figure 17.8 Stream-water chemistry trends at the outlets of a c\earcut watershed (W5)and a reference watershed (W6) at the Hubbard Brook Experimental Forest, NewHampshire.

~ 500-.J~ 400

0

E 300:J

~ 200

'oC') 100Z 0

80-.J

~ 60E

2.- 40

<ir 200';-IJ) 0

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2.- 200() 10000 0

100S"'" 80

E 602.- 40

~o 20() 0

399

0. W5W6

.

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1982 19841983 1985 1988 19891987

Table 17.6 Exchangeable cation pools (kmolc ha-l) before and after whole-tree harvest-ing on W5 at the Hubbard Brook Experimental Forest (after Johnson et aI., 1991)

Forest floor Mineral soil Whole solumElement Before After Before After Before After

Al 1.8 7.7 159.0 160.0 161.0 167.0Ca 3.7 3.7 9.6 12.0 13.0 16.0K 0.7 0.7 2.2 2.4 2.7 3.1Mg 0.4 0.5 2.4 3.6 2.8 4.1H 3.0 2.6 19.0 13.0 22.0 16.0CEC 9.6 15.0 192.0 191.0 202.0 206.0

Reproduced by permission of the author.

400

s

W-S+B-R-P:tE

W - weathering

S - stream loss

B - Net biomass uptake

R - root decay

E - cation exchange

F - Precip + Forest Floor Loss

Calcium(kg ha-l year-I)

13.9

Pre-harvest

51.0

1.8

Post-harvest

Potassium-1 -1

(kg ha year)

2.4

Pre-harvest

0.8

Post-harvest

Figure 17.9 Changes in the budgets of Ca and K in three years following whole-treec1earcuttingat the Hubbard Brook Experimental Forest.


chemistry have been intensively studied. It is believed that increased sulphateadsorption in the mineral soil, and accelerated nitrification in the forest floor causethe observed trends in stream chemistry (Vitousek and Melillo, 1979; Martin etai., 1986; Mann et ai., 1988; Nodvin et ai., 1988).

There was no significant change in the cation exchange capacity (unbufferedNH4Cl extraction) of W5 soils three years after clearcutting (Table 17.6).Exchangeable Ca and K pools increased significantly, while the exchangeable Hpool decreased. Increases in exchangeable Ca and K concentrations were most dra-matic (up to 100%) in the spodic horizons (Bh and Bsl; Johnson et ai., 1991).

The release of Ca and K from decomposing roots was remarkably rapid follow-ing clearcutting. In the first two years 3% of the Ca and 63% of the K in rootbiomass was released. When calculated on an area basis, the input of Ca and K tothe soil solution from root decay averaged 1.8 kg ha-l year-l and 17.3 kg ha-lyear-l for Ca and K, respectively (Fahey et ai., 1988).

The changes in the Ca and K budgets of W5 in the three years following har-vesting are shown in Figure 17.9. These element balances lead to several impor-tant conclusions regarding the biogeochemical reorganization of forest ecosystemsshortly after large-scale disturbance. First, the increased efflux of dissolved cationsin stream water following clearcutting could not be explained by leaching ofexchangeable cations. Indeed, increased retention of exchangeable nutrient cationsin the Bh and Bsl horizons appears to be an important mechanism by which sitefertility is maintained.

Second, the decomposition of roots was an important factor in the reorganiza-tion of nutrients on W5 following logging. Root decay was a significant source ofbasic cations to the soil solution and streamwater. This often overlooked sourcemay partially explain increased export of basic cations at many sites.

After accounting for plant uptake and forest floor mineralization, there remainunexplained losses of Ca and K. It is possible that release of these basic cations bychemical weathering increased following logging. Indeed, soil conditions follow-ing logging-in particular, increased soil moisture and acidity-would seem tofavour this hypothesis. A more complete discussion of this pattern is presented inChapter 14.

17.4.3.2 Factors influencing element export and depletion

A number of edaphic factors and the choice of a management strategy influencethe rate of export of nutrients and other substances following logging. The relativeimportance of any individual factor varies from site to site. Below, we summarizethese factors using selected examples from watershed studies, where possible.


Managementstrategy

The interval of time between harvests ("rotation time") is a critical factor influenc-ing the sustainability of forest cutting practices. Previous catchment studies haveshown that in many systems removal of nutrients in biomass by logging is themost important mechanism of nutrient depletion (Clayton and Kennedy, 1985;Johnson et ai., 1988; Mann et ai., 1988; Federer et ai., 1989).

Mann et ai. (1988) and Federer et ai. (1989) showed that loss of nutrients byleaching was minor in comparison to removal in biomass, with the possible excep-tion of Ca and K. A few watershed studies in the western USA confirm these con-clusions (Clayton and Kennedy, 1985; Martin and Harr, 1989). Johnson et ai.(1988) summarized results from several clearcut watershed and plot studies in theeastern USA. They found that exchangeable base cation supplies in the solumwere not correlated with nutrient export in harvested biomass, but were stronglycorrelated with hydrologic losses of base cations. These results suggest that deple-tion of base cations in the solum due to logging should result in reduced streamwa-ter losses, but continued plant uptake.

The extent of wood fibre utilization can have a profound influence on nutrientdepletion. Much research in this area has centred on the differences between"whole-tree" and "sawlog" or "stem-only" harvesting. Whole-tree harvestingshould have a more adverse impact on forest nutrient cycling because of thegreater biomass export relative to stem-only cutting. Johnson et ai. (1982) con-trasted whole-tree and stem-only harvesting in several watersheds in easternTennessee, USA. They found that export of biomass and nutrients during loggingwas 2.6 to 3.3 times greater under whole-tree harvesting than under stem-only.Several other studies have shown similar results (e.g. Weetman and Webber, 1972;Freedman et ai., 1986; Hendrickson et ai., 1989). Mann et ai. (1988) found nolarge differences in hydrologic export of nutrients between whole-tree harvestedsites and stem-only harvested sites. However, they reported that biomass regrowthon whole-tree harvested sites was slower than on stem-only clearcuts, although thereasons are unclear. It appears that the major difference between the two methodsis in nutrient export in biomass. This difference may be critical in areas with rela-tively small soil pools.

The long-term nature and complexity of predicting the consequences of differ-ent harvest intensities on soil nutrients and forest productivity requires the inte-gration of empirical research and modelling in the context of a general theory.Experimental catchments provide a tool for such assessments as illustrated bySwank and Waide (1980). They coupled extensive nitrogen cycling data with spe-cific process-level research on forested catchments to evaluate changes in soilnitrogen associated with different levels of wood fibre utilization and varying rota-tion ages. Examples of empirical and theoretical models for assessing the futureconsequences of management practices on productivity were reviewed by Yarie(1990).


Vegetation and parent material effects

Vegetation plays a role in nutrient depletion before and after logging. Some treespecies, especially conifers, have low biomass nutrient contents. Logging of thesespecies wilI have less effect on nutrient cycles than removal of other, more nutri-ent-rich species.

Vegetation dynamics following logging also are important to long-term produc-tivity. Regrowing vegetation affects element export in two ways. First, the timingand magnitude of post-harvest water yield increases are controlled by regrowingvegetation as discussed earlier. As transpiration increases with re-establishment ofa forest canopy following logging, discharge decreases. Sites characterized by rapidregrowth or long growing seasons should show lower losses of dissolved nutrientsthan sites where revegetation is slow. Second, regrowing vegetation assimilatesnutrients in biomass, keeping them in the ecosystem. Several studies have shownthat early successional species often have high nutrient concentrations (e.g. Marksand Bormann, 1972; Boring et al., 1981). As these species are outgrown by climaxspecies, their nutrients are recycled, largely remaining in the ecosystem.

Parent material influences long-term productivity through weathering, which isthe major source of soluble nutrient cations in many systems. Through reactionsbetween the soil solution and the exchange complex, nutrient cations released byweathering can serve to replenish depleted exchangeable cations. Weathering con-siderations are discussed in Chapters 4 and 5.

The potential importance of weathering as a nutrient replacement process can beilIustrated by an example. Federer et al. (1989) predicted that 62% of total Ca in ahardwood forest in Connecticut, USA, would be depleted after three 4O-yearrota-tions. If weathering rates at this site are slow, it is possible that the ability of thesoil minerals to supply Ca will not match exports after only one or two rotations.This may result in depletion of the exchangeable Ca pool and adverse effects onvegetation, even though a substantial total pool remains. A few recent studies offersome hope that independent weathering estimates will soon be obtainable(Sverdrup and Warfvinge, 1988; Aberg et aI., 1989).

17.4.3.3 Tropical vs. temperate systems

A major underlying theme of this volume has been the need for more small water-shed studies in tropical and subtropical environments. Contrasting the effects offorest management on element cycles in tropical and temperate systems is difficultbecause of the sparsity of data from tropical watersheds. However, Allen (1985)summarized data from 26 studies of forest clearing effects on soil properties in thetropics and the USA. Results indicate that depletion of N and exchangeable Ca,Mg and K was up to 50% greater on soils developed in highly weathered parentmaterials in the tropics than in similar soils in temperate regions or tropical soilsdeveloped in younger parent materials.A comparisonof soil chemistry in uncut areas


showedthatsoils in olderparentmaterialsin thetropicshadlower concentrationsof N and exchangeable base cations than tropical soils in general (Allen, 1985).

Allen's (1985) data are consistent with depletion patterns outlined above. Ifexport in cut biomass is independent of soil pools, depletion of nutrients should bemore dramatic in areas with small soil pools prior to harvesting, such as the oldertropical soils. Allen's data lead to the tentative hypothesis that differences existbetween the responses of temperate and tropical forests to cutting, and that sometropical forests are more sensitive to cutting than temperate forests. However, asAllen (1985) points out, there is a great deal of variability in the response to cut-ting within the tropics (and temperate systems as well). In order to adequately esti-mate nutrient depletion rates in the tropics, data on hydrologic outputs from loggedtropical watersheds are needed. The sustainability of forestry practices in the trop-ics depends on the degree to which losses from the soil are retained in the ecosys-tem. The establishment of diverse small watershed studies in the tropics is an acuteneed at the present time.

17.5 RESEARCH NEEDS AND OPPORTUNITIES

Small catchment research will continue to provide a primary method for assessingthe effects of forest management practices on the water resource. Principles andguidelines for improved management are a major benefit of catchment-orientedexperimentation. To utilize more fully the results from catchment studies, there isa need to organize and conduct more research from an integrated, interdisciplinaryecosystem approach that is based on theoretical considerations. Understanding ofcause-and-effect relationships derived from such an approach can be formulatedinto models that provide a basis for scaling up and extrapolating results from smallcatchments to larger units of landscapes.

There are many opportunities to use small catchment research to address majorenvironmental issues. The long-term records for baseline forest ecosystems athydrologic stations provide a solid, unique basis for examining changes in foresthealth and water quality from the perspectives of primary pollutants and climatechange. There are also opportunities to expand the small catchment researchapproach to assess other land-use activities such as sewage sludge disposal on for-est lands and intensive recreational use.

Additionally, there is a need to establish catchment research programmes in avariety of forest ecosystems. Historically, most small catchment experimentationhas been conducted in temperate forests and there is a paucity of parallel under-standing for other major biomes such as tropical forests.

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17 small catchment research in the evaluation and ......reviewed by keller (1988) and the evaluation...

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